Chromium coating is widely used as a surface coating for metal articles because of its high hardness value, attractive appearance and superior wear and corrosion resistance. Traditionally, chromium deposition is accomplished by electrodeposition from a chromium plating bath containing hexavalent chromium ions as a source of chromium. The process is highly toxic in nature. Lots of efforts have been made to develop alternative coatings and coating processes to replace the use of hexavalent chromium in electroplating. Among those alternative processes, trivalent chromium electroplating seems to be attractive due to convenience of fabrication through the use of environmental friendly and non-toxic chemicals and ability to produce a bright chromium deposit. However, an industrial scale process giving a hard and corrosion resistant chromium deposit through an aqueous trivalent chromium solution is still missing. Among the industry, there is a hectic need for a well manageable and easy to use chromium based coating process to replace the current use of hexavalent chromium in coating.
Decorative chrome is designed to be aesthetically pleasing and durable. The thickness of decorative chromium coating is generally between 0.05 and 0.5 μm. There has been a strong movement away from hexavalent decorative chromium baths to new trivalent chromium baths. The trivalent form of chromium is considered to be less toxic.
Hard chrome is used to reduce friction, improve durability through abrasion tolerance and wear resistance, minimize galling or seizing of parts, expand chemical inertness to include a broader set of conditions, and as bulking material for worn parts to restore their original dimensions. Hard chromium coatings tend to be thicker than decorative chromium coatings. The thickness of hard chrome can be as high as 200-600 μm. Due to its thickness, the hardness of hard chrome is usually over 700 HV. Today, hard chrome is almost exclusively electroplated from hexavalent chromium baths because of difficulties in reaching desired wear resistance and hardness by using trivalent chromium baths.
Many chromium plating processes of prior art are not capable of producing coatings with a Vickers microhardness value of 2000 HV or more. Further defects of the known chromium-based coatings are their inadequate wear and corrosion resistances. Chromium coating as such is very brittle in character. The number of cracks and microcracks in a chromium coating increases together with the thickness of the coating, thus impairing the corrosion resistance of the coating.
Deposition of nickel, either by electroless plating or electroplating, has also been proposed as an alternative to hard chrome. Drawbacks of nickel plating include deficiencies in hardness, friction coefficient, wear resistance, corrosion resistance and adhesion. Nickel plating and hard chrome are not interchangeable coatings. The two have unique deposit properties and, therefore, each has its distinct applications.
In the prior art, several attempts have been made to improve the corrosion resistance of an object by multi-layer coating. However, relatively little success has been reported on improvement of abrasive wear resistance produced by multi-layer coating.
U.S. Pat. No. 2,859,158 discloses a process for coating molybdenum with a nickel-chromium diffusion alloy. The process consists of depositing sequentially a layer of chromium and a layer of nickel, repeating said sequence of depositions a plurality of times, and heating the coated molybdenum for 4 hours at a temperature above 980° C. and substantially below the melting point of the eutectic of the metals. Chromium layers are deposited from hexavalent chromium bath. The use of hexavalent chromium in electroplating is something that should be avoided today. The heat treatment carried out after coating is quite harsh. The method is suitable only for coating of molybdenum.
SE 205488 discloses a coating for an article of ferroalloy, comprising alternating layers of nickel and chromium, the lowermost layer being of nickel and the topmost layer being either of nickel or chromium. Chromium is deposited from hexavalent chromium containing electroplating bath.
US 2010/0025255 discloses an electroplating method for magnesium and magnesium alloy substrate. The method comprises chemically plating the substrate to form a nickel coating on its surface and electroplating the substrate to form, in order, a first nickel coating, a copper coating, a second nickel coating and a chromium coating on the chemically produced nickel coating.
Apparently, there is a need for a chromium-based coating which is able to yield such utmost mechanical properties that enable replacement of hexavalent chromium baths.